Many variants of this type of equipment exist already. They include an optical probe designed to be placed on a region of the skin of a user. The probe includes an emitter for radiating light into the tissue of that region and a receiver for picking up energy coming from the tissue (for example as a result of reflection on the tissue or transmission through the tissue) whose optical characteristics vary in dependence upon the circulation of the blood. The signal supplied by the receiver is analyzed and artifacts caused in particular by the movements of the user, ambient light, etc. are removed in an electronic processing unit whose output signal can be made perceptible in any appropriate manner, for example by displaying it on a screen, by reproducing it audibly, etc. The output signal can be compared to a threshold and an alarm tripped if the heart rate crosses the threshold.
Equipment having the above features is described in EP 01203686.9, filed Sep. 28, 2001 and entitled “Wrist located pulse detection using infra-red signals, activity and nonlinear artefact cancellation”. That document describes in particular a probe intended to be worn on the wrist, the useful signal being extracted by a device for detecting movements of the wearer.
In another prior art device, the probe is incorporated into a chest strap and the useful signal is transmitted by radio to a display device, such as a wristwatch.
An object of the invention is to provide equipment of the kind indicated above that provides reliable information concerning the heart rate of the wearer whilst being convenient to use and particularly suitable for use by persons exercising an activity, for example a sports activity.